Method for detecting foreign object, wireless power transfer control apparatus, and wireless power transfer apparatus using the same

ABSTRACT

A method for detecting a foreign object between a transmission pad and a reception pad for wireless power transfer (WPT) may comprise steps of performing a zero phase angle (ZPA) control for a power transfer converter included in or connected to the transmission pad; detecting a characteristic parameter with respect to at least one of an input current and a switching frequency of the power transfer converter; and determining whether or not a foreign object exists between the transmission pad and the reception pad according to whether the characteristic parameter with respect to at least one of the input current and the switching frequency deviates from an allowable range.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on and claims the benefit of priority to Korean Patent Application No. 10-2017-0112726 filed on Sep. 4, 2017 with the Korean Intellectual Property Office, the entire contents of which are incorporated herein for all purposes by this reference.

TECHNICAL FIELD

The present disclosure relates to a method for detecting a foreign object, and a wireless power transfer (WPT) control apparatus and a WPT apparatus using the same, more particularly, to a method for detecting a foreign object between a transmission pad and a reception pad for WPT, and a WPT control apparatus and a WPT apparatus using the same.

BACKGROUND

An electric vehicle (EV) charging system may basically be defined as a system for charging a high-voltage battery mounted on an EV by using power of an energy storage device or a power grid of a commercial power source. Such an EV charging system may have various forms according to a type of EV. For example, the EV charging system may be classified into a conductive type using a charging cable and a non-contact wireless power transfer (WPT) type (also referred to as an ‘inductive type’).

In the case of an inductive type charging using a WPT system, when a high-voltage battery mounted on the EV is to be charged, the EV may be positioned to a ground assembly (GA) located in a charging station or a charging spot capable of EV charging.

Especially, a magnetic induction type of the WPT system uses an electromagnetic induction phenomenon between a transmission coil in the GA and a reception coil in a vehicle assembly to transfer electric power. When charging the EV, the reception coil in the VA mounted on the EV makes an inductive resonance coupling with the transmission coil in the GA located in the charging station or the charging spot, and charges the battery in the EV using power transferred from the GA through the inductive resonance coupling.

However, when a foreign object, such as metal or magnetic material, which may affect a magnetic field, exists between the transmission coil and the reception coil, there is a direct influence on the resonant network of the WPT system, thereby causing abnormal operations of the system. Also, due to the presence of the foreign object, a temperature of the foreign object may sharply increase, causing a problem in the system stability.

SUMMARY

Embodiments of the present disclosure provide a method for detecting a foreign object between a transmission pad and a reception pad for WPT.

Embodiments of the present disclosure also provide a WPT control apparatus using the method for detecting a foreign object.

Embodiments of the present disclosure also provide a WPT apparatus including the WPT control apparatus.

According to embodiments of the present disclosure, a method for detecting a foreign object between a transmission pad and a reception pad for wireless power transfer (WPT) may comprise steps of performing a zero phase angle (ZPA) control for a power transfer converter included in or connected to the transmission pad; detecting a characteristic parameter with respect to at least one of an input current and a switching frequency of the power transfer converter; and determining whether or not a foreign object exists between the transmission pad and the reception pad according to whether the characteristic parameter with respect to at least one of the input current and the switching frequency deviates from an allowable range.

The characteristic parameter may be a change rate of a moving average of the switching frequency of the power transfer converter.

In the step of determining, the foreign object may be determined to exist between the transmission pad and the reception pad when the change rate of the moving average of the switching frequency is equal to or greater than a first threshold.

The moving average may be a simple moving average, a weighted moving average, or an exponential moving average.

The characteristic parameter may be a moving average of the input current, and the input current may be sensed at a current sensing point at which a zero current switching (ZCS) operation is performed.

In the step of determining, the foreign object may be determined to exist between the transmission pad and the reception pad when the moving average of the input current deviates from the allowable range.

The method may further comprise a step of, in response to determining that the foreign object exists between the transmission pad and the reception pad, controlling the power transfer converter to make the WPT be stopped.

The ZPA control may be a control that causes an input voltage and the input current of the power transfer converter to be in-phase.

Furthermore, in accordance with embodiments of the present disclosure, a wireless power transfer (WPT) control apparatus may comprise at least one processor and a non-transitory memory storing at least one instruction executed by the at least one processor. Also, execution of the at least one instruction in the non-transitory memory may cause the at least one processor to perform a zero phase angle (ZPA) control for a power transfer converter included in or connected to a transmission pad; detect a characteristic parameter with respect to at least one of an input current and a switching frequency of the power transfer converter; and determine whether or not a foreign object exists between the transmission pad and a reception pad according to whether the characteristic parameter with respect to at least one of the input current and the switching frequency deviates an allowable range.

The characteristic parameter may be a change rate of a moving average of the switching frequency of the power transfer converter.

The foreign object may be determined to exist between the transmission pad and the reception pad when the change rate of the moving average of the switching frequency is equal to or greater than a first threshold.

The moving average may be a simple moving average, a weighted moving average, or an exponential moving average.

The characteristic parameter may be a moving average of the input current.

The foreign object may be determined to exist between the transmission pad and the reception pad when the moving average of the input current deviates from the allowable range.

The at least one processor may be further configured to control the power transfer converter to make the WPT be stopped in response to determining that the foreign object exists between the transmission pad and the reception pad.

Furthermore, in accordance with embodiments of the present disclosure, a wireless power transfer (WPT) apparatus may comprise a power transfer converter included in or connected to a transmission pad for WPT, the power transfer converter converting an input voltage to an output voltage in an induction power transfer (IPT) manner, and outputting the output voltage; at least one processor; and a non-transitory memory storing at least one instruction executed by the at least one processor. Also, execution of the at least one instruction in the non-transitory memory may cause the at least one processor to perform a zero phase angle (ZPA) control for the power transfer converter; detect a characteristic parameter with respect to at least one of an input current and a switching frequency of the power transfer converter; and determine whether or not a foreign object exists between the transmission pad and the reception pad according to whether the characteristic parameter with respect to at least one of the input current and the switching frequency deviates an allowable range.

The characteristic parameter may be a change rate of a moving average of the switching frequency of the power transfer converter.

The foreign object may be determined to exist between the transmission pad and the reception pad when the change rate of the moving average of the switching frequency is equal to or greater than a first threshold.

The moving average may be a simple moving average, a weighted moving average, or an exponential moving average.

The at least one processor may be further configured to control the power transfer converter to make the WPT to be stopped in response to determining that the foreign object exists between the transmission pad and the reception pad.

Using the foreign object detecting method, the WPT control apparatus, and the WPT apparatus according to the embodiments of the present disclosure as described above, it is made possible to detect a foreign object between the transmission and reception pads for WPT without adding any hardware. Also, a foreign object such as metal can be detected during operation of the WPT apparatus.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present disclosure will become more apparent by describing in detail embodiments of the present disclosure with reference to the accompanying drawings, in which:

FIG. 1 is a conceptual diagram illustrating a concept of a wireless power transfer (WPT) to which embodiments of the present disclosure are applied;

FIG. 2 is a conceptual diagram for explaining a concept of alignment in an EV WPT according to embodiments of the present disclosure;

FIG. 3 is a conceptual diagram illustrating a WPT circuit according to embodiments of the present disclosure;

FIG. 4 is a detailed circuit diagram illustrating an inductive power transfer (IPT) converter to which embodiments of the present disclosure are applied;

FIG. 5 is a flowchart for explaining a quasi-ZPA control method of an IPT converter;

FIG. 6A is a graph illustrating waveforms of an input current and an input voltage in an IPT converter in a normal case, and FIG. 6B is a graph illustrating waveforms of an output voltage and a current induced at a secondary coil in the normal case;

FIG. 7A is a graph illustrating waveforms of an input current and an input voltage for an IPT converter in a case that a foreign object exists, and FIG. 7B is a graph illustrating waveforms of an output voltage and a current induced at a secondary coil in the case that a foreign object exists;

FIG. 8 is a flowchart for explaining a method of detecting a foreign object between a transmission coil and a reception coil according to an embodiment of the present disclosure;

FIG. 9 is a conceptual diagram for explaining a method of detecting a foreign object between a transmission coil and a reception coil according to another embodiment of the present disclosure;

FIG. 10 is a flowchart for explaining a method of detecting a foreign object between a transmission coil and a reception coil according to another embodiment of the present disclosure; and

FIG. 11 is a block diagram illustrating a WPT apparatus according to an embodiment of the present disclosure.

It should be understood that the above-referenced drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the disclosure. The specific design features of the present disclosure, including, for example, specific dimensions, orientations, locations, and shapes, will be determined in part by the particular intended application and use environment.

DETAILED DESCRIPTION

Embodiments of the present disclosure are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing embodiments of the present disclosure, however, embodiments of the present disclosure may be embodied in many alternate forms and should not be construed as limited to embodiments of the present disclosure set forth herein. While describing the respective drawings, like reference numerals designate like elements.

It will be understood that although the terms “first,” “second,” etc. may be used herein to describe various components, these components should not be limited by these terms. These terms are used merely to distinguish one element from another. For example, without departing from the scope of the present disclosure, a first component may be designated as a second component, and similarly, the second component may be designated as the first component. The term “and/or” include any and all combinations of one of the associated listed items.

It will be understood that when a component is referred to as being “connected to” another component, it can be directly or indirectly connected to the other component. That is, for example, intervening components may be present. On the contrary, when a component is referred to as being “directly connected to” another component, it will be understood that there is no intervening components.

Terms are used herein only to describe the embodiments but not to limit the present disclosure. Singular expressions, unless defined otherwise in contexts, include plural expressions. In the present specification, terms of “comprise” or “have” are used to designate features, numbers, steps, operations, elements, components or combinations thereof disclosed in the specification as being present but not to exclude possibility of the existence or the addition of one or more other features, numbers, steps, operations, elements, components, or combinations thereof.

All terms including technical or scientific terms, unless being defined otherwise, have the same meaning generally understood by a person of ordinary skill in the art. It will be understood that terms defined in dictionaries generally used are interpreted as including meanings identical to contextual meanings of the related art, unless definitely defined otherwise in the present specification, are not interpreted as being ideal or excessively formal meanings.

Terms used in the present disclosure are defined as follows.

“Electric Vehicle, EV”: An automobile, as defined in 49 CFR 523.3, intended for highway use, powered by an electric motor that draws current from an on-vehicle energy storage device, such as a battery, which is rechargeable from an off-vehicle source, such as residential or public electric service or an on-vehicle fuel powered generator. The EV may be four or more wheeled vehicle manufactured for use primarily on public streets, roads.

The EV may be referred to as an electric car, an electric automobile, an electric road vehicle (ERV), a plug-in vehicle (PV), a plug-in vehicle (xEV), etc., and the xEV may be classified into a plug-in all-electric vehicle (BEV), a battery electric vehicle, a plug-in electric vehicle (PEV), a hybrid electric vehicle (HEV), a hybrid plug-in electric vehicle (HPEV), a plug-in hybrid electric vehicle (PHEV), etc.

“Plug-in Electric Vehicle, PEV”: An Electric Vehicle that recharges the on-vehicle primary battery by connecting to the power grid.

“Plug-in vehicle, PV”: An electric vehicle rechargeable through wireless charging from an electric vehicle supply equipment (EVSE) without using a physical plug or a physical socket.

“Heavy duty vehicle; H.D. Vehicle”: Any four- or more wheeled vehicle as defined in 49 CFR 523.6 or 49 CFR 37.3 (bus).

“Light duty plug-in electric vehicle”: A three or four-wheeled vehicle propelled by an electric motor drawing current from a rechargeable storage battery or other energy devices for use primarily on public streets, roads and highways and rated at less than 4,545 kg gross vehicle weight.

“Wireless power charging system, WCS”: The system for wireless power transfer and control between the GA and VA including alignment and communications. This system transfers energy from the electric supply network to the electric vehicle electromagnetically through a two-part loosely coupled transformer.

“Wireless power transfer, WPT”: The transfer of electrical power from the AC supply network to the electric vehicle by contactless means.

“Utility”: A set of systems which supply electrical energy and may include a customer information system (CIS), an advanced metering infrastructure (AMI), rates and revenue system, etc. The utility may provide the EV with energy through rates table and discrete events. Also, the utility may provide information about certification on EVs, interval of power consumption measurements, and tariff.

“Smart charging”: A system in which EVSE and/or PEV communicate with power grid in order to optimize charging ratio or discharging ratio of EV by reflecting capacity of the power grid or expense of use.

“Automatic charging”: A procedure in which inductive charging is automatically performed after a vehicle is located in a proper position corresponding to a primary charger assembly that can transfer power. The automatic charging may be performed after obtaining necessary authentication and right.

“Interoperability”: A state in which components of a system interwork with corresponding components of the system in order to perform operations aimed by the system. Also, information interoperability may mean capability that two or more networks, systems, devices, applications, or components can efficiently share and easily use information without causing inconvenience to users.

“Inductive charging system”: A system transferring energy from a power source to an EV through a two-part gapped core transformer in which the two halves of the transformer, primary and secondary coils, are physically separated from one another. In the present disclosure, the inductive charging system may correspond to an EV power transfer system.

“Inductive coupler”: The transformer formed by the coil in the GA Coil and the coil in the VA Coil that allows power to be transferred with galvanic isolation.

“Inductive coupling”: Magnetic coupling between two coils. The “inductive coupling” in the present disclosure refers to a coupling between the GA Coil and the VA Coil.

“Ground assembly, GA′”: An assembly on the infrastructure side consisting of the GA Coil, a power/frequency conversion unit and GA controller as well as the wiring from the grid and between each unit, filtering circuits, housing(s) etc., necessary to function as the power source of wireless power charging system. The GA may include the communication elements necessary for communication between the GA and the VA.

“Vehicle assembly, VA”: An assembly on the vehicle consisting of the VA Coil, rectifier/power conversion unit and VA controller as well as the wiring to the vehicle batteries and between each unit, filtering circuits, housing(s), etc., necessary to function as the vehicle part of a wireless power charging system. The VA may include the communication elements necessary for communication between the VA and the GA.

The GA may be referred to as a primary device (PD), and the VA may be referred to as a secondary device (SD).

“Primary device”: An apparatus which provides the contactless coupling to the secondary device. That is, the primary device may be an apparatus external to an EV. When the EV is receiving power, the primary device may act as the source of the power to be transferred. The primary device may include the housing and all covers.

“Secondary device”: An apparatus mounted on the EV which provides the contactless coupling to the primary device. That is, the secondary device may be installed in the EV. When the EV is receiving power, the secondary device may transfer the power from the primary to the EV. The secondary device may include the housing and all covers.

“GA controller”: The portion of the GA which regulates the output power level to the GA Coil based on information from the vehicle.

“VA controller”: The portion of the VA that monitors specific on-vehicle parameters during charging and initiates communication with the GA to control output power level.

The GA controller may be referred to as a primary device communication controller (PDCC), and the VA controller may be referred to as an electric vehicle communication controller (EVCC).

“Magnetic gap”: The vertical distance between the plane of the higher of the top of the litz wire or the top of the magnetic material in the GA Coil to the plane of the lower of the bottom of the litz wire or the magnetic material in the VA Coil when aligned.

“Ambient temperature”: The ground-level temperature of the air measured at the subsystem under consideration and not in direct sun light.

“Vehicle ground clearance”: The vertical distance between the ground surface and the lowest part of the vehicle floor pan.

“Vehicle magnetic ground clearance”: The vertical distance between the plane of the lower of the bottom of the litz wire or the magnetic material in the VA Coil mounted on a vehicle to the ground surface.

“VA Coil magnetic surface distance”: the distance between the plane of the nearest magnetic or conducting component surface to the lower exterior surface of the VA coil when mounted. This distance includes any protective coverings and additional items that may be packaged in the VA Coil enclosure.

The VA coil may be referred to as a secondary coil, a vehicle coil, or a receive coil. Similarly, the GA coil may be referred to as a primary coil, or a transmit coil.

“Exposed conductive component”: A conductive component of electrical equipment (e.g. an electric vehicle) that may be touched. The conductive component may not be normally energized but which may become energized in case of a fault.

“Hazardous live component”: A live component, which under certain conditions can give a harmful electric shock.

“Live component”: Any conductor or conductive component intended to be electrically energized in normal use.

“Direct contact”: Contact of persons with live components. (See IEC 61440)

“Indirect contact”: Contact of persons with exposed, conductive, and energized components made live by an insulation failure. (See IEC 61140)

“Alignment”: A process of finding the relative position of primary device to secondary device and/or finding the relative position of secondary device to primary device for the efficient power transfer that is specified. In the present disclosure, the alignment may direct to a fine positioning of the wireless power transfer system.

“Pairing”: A process by which a vehicle is correlated with the unique dedicated primary device, at which it is located and from which the power will be transferred. Pairing may include the process by which a VA controller and a GA controller of a charging spot are correlated. The correlation/association process may include the process of establishment of a relationship between two peer communication entities.

“Command and control communication”: The communication between the EV supply equipment and the EV exchanges information necessary to start, control and terminate the process of WPT.

“High level communication (HLC)”: HLC is a special kind of digital communication. HLC is necessary for additional services which are not covered by command & control communication. The data link of the HLC may use a power line communication (PLC), but it is not limited thereto.

“Low power excitation (LPE)”: LPE means a technique of activating the primary device for the fine positioning and pairing so that the EV can detect the primary device, and vice versa.

“Service set identifier (SSID)”: SSID is a unique identifier consisting of 32-characters attached to a header of a packet transmitted on a wireless LAN. The SSID identifies the basic service set (BSS) to which the wireless device attempts to connect. The SSID distinguishes multiple wireless LANs. Therefore, all access points (APs) and all terminal/station devices that want to use a specific wireless LAN can use the same SSID. Devices that do not use a unique SSID are not able to join the BSS. Since the SSID is shown as plain text, it may not provide any security features to the network.

“Extended service set identifier (ESSID)”: ESSID is the name of the network to which one desires to connect. It is similar to SSID but can be a more extended concept.

“Basic service set identifier (BSSID)”: BSSID consisting of 48 bits is used to distinguish a specific BSS. In the case of an infrastructure BSS network, the BSSID may be medium access control (MAC) of the AP equipment. For an independent BSS or ad hoc network, the BSSID can be generated with any value.

The charging station may comprise at least one GA and at least one GA controller configured to manage the at least one GA. The GA may comprise at least one wireless communication device. The charging station may mean a place having at least one GA, which is installed in home, office, public place, road, parking area, etc.

Additionally, it is understood that one or more of the below methods, or aspects thereof, may be executed by at least one controller. The term “controller” may refer to a hardware device that includes a non-transitory memory and a processor. The non-transitory memory is configured to store program instructions, and the processor is specifically programmed to execute the program instructions to perform one or more processes which are described further below. Moreover, it is understood that the below methods may be executed by an apparatus comprising the controller in conjunction with one or more other components, as would be appreciated by a person of ordinary skill in the art.

In embodiments according to the present disclosure, a light load driving or light load operation may include, for example, charging a high voltage battery with a charging voltage lower than a predetermined rated voltage in the latter half of charging for the high voltage battery connected to the VA in the WPT system. Also, the light load operation may include a case in which the high-voltage battery of EV is charged at a relatively low voltage and at a low speed by using a low-speed charger such as a household charger.

Hereinafter, embodiments according to the present disclosure will be explained in detail by referring to accompanying figures.

FIG. 1 is a conceptual diagram illustrating a concept of a wireless power transfer (WPT) to which embodiments of the present disclosure are applied.

Referring to FIG. 1, a WPT may be performed by at least one component of an electric vehicle (EV) 10 and a charging station 13, and may be used for wirelessly transferring power to the EV 10.

Here, the EV 10 may be usually defined as a vehicle supplying an electric power stored in a rechargeable energy storage including a battery 12 as an energy source of an electric motor which is a powertrain system of the EV 10.

The EV 10 according to an embodiment of the present disclosure may include a hybrid electric vehicle (HEV) having an electric motor and an internal combustion engine together, and may include not only an automobile but also a motorcycle, a cart, a scooter, and an electric bicycle.

Further, the EV 10 may include a power reception pad 11 including a reception coil for charging the battery 12 wirelessly and may include a plug connection for conductively charging the battery 12. Here, the EV 10 configured for conductively charging the battery 12 may be referred to as a plug-in electric vehicle (PEV).

The charging station 13 may be connected to a power grid 15 or a power backbone, and may provide an alternating current (AC) power or a direct current (DC) power to a power transmission pad 14 including a transmission coil through a power link.

Further, the charging station 13 may communicate with an infrastructure management system or an infrastructure server that manages the power grid 15 or a power network through wired/wireless communications, and performs wireless communications with the EV 10.

Here, the wireless communications may be Bluetooth, Zigbee, cellular, wireless local area network (WLAN), or the like.

The charging station 13 may be located at various places including, for example, a parking area attached to the owner's house of the EV 10, a parking area for charging an EV at a gas station, a parking area at a shopping center or a workplace.

A process of wirelessly charging the battery 12 of the EV 10 may begin with placing the power reception pad 11 of the EV 10 in an energy field generated by the power transmission pad 14 of the charging station 13, and making the reception coil and the transmission coil be interacted or coupled to each other. An electromotive force may be induced in the power reception pad 11 as a result of the interaction or coupling, and the battery 12 may be charged by the induced electromotive force.

The charging station 13 and the transmission pad 14 may be referred to as a ground assembly (GA) in whole or in part, where the GA may refer to the previously defined meaning.

All or part of the internal components and the reception pad 11 of the EV 10 may be referred to as a vehicle assembly (VA), which may refer to the previously defined meaning. Here, the power transmission pad 14 or the power reception pad 11 may be configured to be non-polarized or polarized.

In a case that a pad is non-polarized, there is one pole in a center of the pad and an opposite pole in an external periphery. Here, a flux may be formed to exit from the center of the pad and return at all to external boundaries of the pad.

In a case that a pad is polarized, it may have a respective pole at either end portion of the pad. Here, a magnetic flux may be formed based on an orientation of the pad.

FIG. 2 is a conceptual diagram for explaining a concept of alignment in an EV WPT according to embodiments of the present disclosure.

Referring to FIG. 2, a method of aligning the power transmission pad 14 and the power reception pad 11 in the EV 10 in FIG. 1 will be described. Here, a positional alignment may correspond to the alignment, which is the above-mentioned term, and thus may be defined as a positional alignment between the GA and the VA, but is not limited to the alignment of the transmission pad 14 and the reception pad 11.

Although the transmission pad 14 is illustrated as positioned below a ground surface as shown in FIG. 2, the transmission pad 14 may also be positioned on the ground surface, or positioned such that a top portion surface of the transmission pad 14 is exposed below the ground surface.

The reception pad 11 of the EV 10 may be defined by different categories according to its heights (defined in the z direction) measured from the ground surface. For example, a class 1 for reception pads having a height of 100-150 millimeters (mm) from the ground surface, a class 2 for reception pads having a height of 140-210 mm, and a class 3 for reception pads having a height of 170-250 mm may be defined. Here, the reception pad may support a part of the above-described classes 1 to 3. For example, only the class 1 may be supported according to the type of the reception pad 11, or the class 1 and 2 may be supported according to the type of the reception pad 11.

Here, the height of the reception pad 11 measured from the ground surface may correspond to the previously defined term ‘vehicle magnetic ground clearance’.

Further, the position of the power transmission pad 14 in the height direction (i.e., defined in the z direction) may be determined to be located between the maximum class and the minimum class supported by the power reception pad 11. For example, when the reception pad supports only the class 1 and 2, the position of the power transmission pad 14 may be determined between 100 and 210 mm with respect to the power reception pad 11.

Still further, a gap between the center of the power transmission pad 14 and the center of the power reception pad 11 may be determined to be located within the limits of the horizontal and vertical directions (defined in the x and y directions). For example, it may be determined to be located within ±75 mm in the horizontal direction (defined in the x direction), and within ±100 mm in the vertical direction (defined in the y direction).

Here, the relative positions of the power transmission pad 14 and the power reception pad 11 may be varied in accordance with their experimental results, and the numerical values should be understood as exemplary.

FIG. 3 is a conceptual diagram illustrating a WPT circuit according to embodiments of the present disclosure.

Referring to FIG. 3, a schematic configuration of a circuit in which a WPT is performed in an EV WPT system may be seen.

Here, the left-hand side of FIG. 3 may be interpreted as expressing all or part of a power source V_(src) supplied from the power network, the charging station 13, and the transmission pad 14 in FIG. 1, and the right-hand side of FIG. 3 may be interpreted as expressing all or part of the EV 10 including the reception pad 11 and the battery 12.

First, the circuit of left-hand side of FIG. 3 may provide an output power P_(src) corresponding to the power source V_(src) supplied from the power network to a primary-side power converter. The primary-side power converter may supply an output power P₁ converted from the output power P_(src) through frequency-converting and AC-to-DC/DC-to-AC converting to generate an electromagnetic field at a desired operating frequency in a transmission coil L₁.

Specifically, the primary-side power converter may include an AC/DC converter for converting the power P_(src) which is an AC power supplied from the power network into a DC power, and a low frequency (LF) converter for converting the DC power into an AC power having an operating frequency suitable for wireless charging. For example, the operating frequency for wireless charging may be determined to be within 80 to 90 kHz.

The power P₁ output from the primary-side power converter may be supplied again to a circuit including the transmission coil L₁, a first capacitor C₁ and a first resistor R₁. Here, a capacitance of the first capacitor C₁ may be determined as a value to have an operating frequency suitable for charging together with the transmission coil L₁. Here, the first resistor R₁ may represent a power loss occurred by the transmission coil L₁ and the first capacitor C₁.

Further, the transmission coil L₁ may be made to have electromagnetic coupling, which is defined by a coupling coefficient m, with the reception coil L₂ so that a power P₂ is transmitted, or the power P₂ is induced in the reception coil L₂. Therefore, the meaning of power transfer in the present disclosure may be used together with the meaning of power induction.

Still further, the power P₂ induced in or transferred to the reception coil L₂ may be provided to a secondary-side power converter. Here, a capacitance of a second capacitor C₂ may be determined as a value to have an operating frequency suitable for wireless charging together with the reception coil L₂, and a second resistor R₂ may represent a power loss occurred by the reception coil L₂ and the second capacitor C₂.

The secondary-side power converter may include an LF-to-DC converter that converts the supplied power P₂ of a specific operating frequency to a DC power having a voltage level suitable for the battery V_(HV) of the EV 10.

The electric power P_(HV) converted from the power P₂ supplied to the secondary-side power converter may be output, and the power P_(HV) may be used for charging the battery V_(HV) disposed in the EV 10.

Here, the circuit of right-hand side of FIG. 3 may further include a switch for selectively connecting or disconnecting the reception coil L₂ with the battery V_(HV). Here, resonance frequencies of the transmission coil L₁ and the reception coil L₂ may be similar or identical to each other, and the reception coil L₂ may be positioned near the electromagnetic field generated by the transmission coil L₁.

Here, the circuit of FIG. 3 should be understood as an illustrative circuit for WPT in the EV WPT system used for embodiments of the present disclosure, and is not limited to the circuit illustrated in FIG. 3.

On the other hand, since the power loss may increase as the transmission coil L₁ and the reception coil L₂ are located at a long distance, it may be an important factor to properly set the relative positions of the transmission coil L₁ and the reception coil L₂.

Here, the transmission coil L₁ may be included in the transmission pad 14 in FIG. 1, and the reception coil L₂ may be included in the reception pad 11 in FIG. 1. Therefore, positioning between the transmission pad 14 and the reception pad 11 or positioning between the EV 10 and the transmission pad 14 will be described below with reference to the drawings.

FIG. 4 is a detailed circuit diagram illustrating an inductive power transfer (IPT) converter 400 to which embodiments of the present disclosure are applied.

The IPT converter 400 illustrated in FIG. 4 may be an LCCL-S type converter circuit, and may be an example of various power transfer converter circuits to which embodiments of the present disclosure may be applied.

The IPT converter 400 illustrated in FIG. 4 may be mainly configured to comprise an AC inverter 410, a resonant tank 420, and a rectifier bridge 430. The IPT converter 400 may convert an input voltage V_(in) to an output voltage V_(o) in an IPT manner, transfer the output voltage V_(o) to a battery, and perform constant current/constant voltage charging to the battery under a zero phase angle (ZPA) condition.

The AC inverter 410 may be a circuit including four switches. The AC inverter 410 may convert an input voltage V_(dc_link) in a DC form into a voltage V_(in) in an AC form according to switching operations of the first to fourth switches, and transfer the converted AC voltage to the resonant tank 420.

The resonant tank 420 may be divided into a primary resonant tank and a secondary resonant tank. The primary resonant tank may be connected to the AC inverter 410 and the secondary resonant tank may be connected to the rectifier bridge 430 to perform the voltage conversion through IPT between the primary resonant tank and the secondary resonant tank.

Specifically, the primary resonant tank may comprise a primary resonant inductor L_(in) and primary resonant capacitors C_(f) and C_(p). Here, the resonant capacitors C_(f) and C_(p) may be connected to a primary coil L_(p) in series with each other. The primary resonant tank may be implemented on input voltage lines connecting a pair of legs constituting the AC inverter 410. That is, the primary resonant inductor L_(in) and one end of the resonant capacitor C_(p) may be connected to the AC inverter 410.

The secondary resonant tank may comprise a secondary resonant capacitor C_(s) and may be connected to a secondary coil L_(s). The secondary resonant tank may be implemented on output voltage lines connecting a pair of legs constituting the rectifier bridge 430.

In order to minimize a voltage-current (VA) rating of the IPT converter circuit illustrated in FIG. 4, it is necessary to perform a ZPA control so that the input voltage V_(in) and an input current I_(in) are in-phase.

FIG. 5 is a flowchart for explaining a quasi-ZPA control method of an IPT converter 400.

Referring to FIG. 5, when a quasi-ZPA is started (S501), the input current I_(in) of the IPT converter 400 may be sensed (S502). The sensing of the input current may be continuously performed over time, thereby deriving a moving average I_(in_avg) of the input current (S503).

When the moving average I_(in_avg) of the input current is derived, a switching frequency may be adjusted by comparing the moving average with an upper limit value and a lower limit value of the input current. More specifically, when the moving average I_(in_avg) of the input current exceeds the upper limit value I_(H_bound) (S511), the switching frequency f may be decreased (S521). Here, decreasing the switching frequency may mean increasing a peak value Carr_(peak) of a carrier signal in a form of sawtooth. In the IPT converter 400 illustrated in FIG. 4, gate signals for the switches may be generated in a digital processor (DSP) by comparing the sawtooth wave (i.e., a wave of the carrier signal) with a reference wave, and a maximum value of the sawtooth wave may be expressed by Carr_(peak).

On the other hand, when the moving average I_(in_avg) of the input current is less than the lower limit value I_(L_bound), the switching frequency may be increased (S522). Here, increasing the switching frequency may mean decreasing the peak value Carr_(peak) of the carrier signal in the form of sawtooth.

If the moving average I_(in_avg) of the input current does not exceed the upper limit value I_(H_bound) and is not less than the lower limit value I_(L_bound), that is, if the moving average I_(in_avg) of the input current is between the upper limit value and the lower limit value, the switching frequency f may be maintained (S523).

When the above-described frequency adjustment procedure is completed, it may be determined whether the current moving average I_(in_avg) is less than the I_(ZCS) (S531). If the current moving average I_(in_avg) is less than I_(ZCS), it may be checked whether the battery charging is completed (S541), and the above-described steps (S502 to S541) may be repeatedly performed until the battery charging is completed. When the battery charging is completed, the WPT may be stopped (S551).

On the other hand, if the current moving average I_(in_avg) is equal to or higher than the I_(ZCS) after the frequency adjustment procedure is completed, the WPT may be stopped (S551). Here, I_(ZCS) may mean a current value of a current sensing point at which a zero current switching (ZCS) operation is performed. A zero voltage switching (ZVS, current phase lagging) and a zero current switching (ZCS, current phase leading) may occur according to a phase difference between the input voltage V_(in) and the input current I_(in) which is a resonant current. In order for the WPT system to operate normally, the ZVS operation should be performed. When the magnitude of I_(in) at the current sensing point is larger than 0, it may mean that the ZCS operation is being performed, and thus the WPT may be stopped. The I_(ZCS) may have a preset threshold, for example, a value of 1 A.

In the case of the conventional ZPA control explained with reference to FIG. 5, phases of the input voltage and the input current are detected, and the ZPA operation is performed through the variable control on the switching frequency. When the IPT converter 400 reaches a steady state, the switching frequency may be fixed at a constant value, and a frequency fluctuation width due to factors such as load variation is small.

FIG. 6A is a graph illustrating waveforms of an input current and an input voltage in an IPT converter 400 in a normal case, and FIG. 6B is a graph illustrating waveforms of an output voltage and a current induced at a secondary coil in the normal case.

FIG. 6A illustrates a relation between the input voltage Vu of the IPT converter 400 and the resonant current I_(p) of the primary coil in the primary resonant tank in a normal case. A pulse-shaped signal represents the input voltage V_(in), and a sinusoidal wave signal represents the resonant current I_(p). Here, the normal case may mean that no foreign object is interposed between the transmission coil and the reception coil.

Referring to FIG. 6A, it may be seen that a phase difference occurs between the input voltage V_(in) and the resonant current I_(p). That is, the input voltage and the resonant current illustrated in FIG. 6A indicate the input voltage and the resonant current for the IPT converter 400 in which the ZPA control is not completely performed. If the ZPA control is perfectly performed under the condition that no foreign object exists between the transmission coil and the reception coil, the phase difference between the input voltage and the resonant current may not occur.

FIG. 6B illustrates a relation between the output voltage V_(o,ac) and a current I_(s) induced at the secondary coil in the normal case.

FIG. 7A is a graph illustrating waveforms of an input current and an input voltage for an IPT converter 400 in a case that a foreign object exists, and FIG. 7B is a graph illustrating waveforms of an output voltage and a current induced at a secondary coil in the case that a foreign object exists.

The graph illustrated in FIG. 7A is a graph in comparison with FIG. 6A, which illustrates a relation between the input voltage and the output voltage in the presence of a foreign object that may affect a magnetic field, such as metal or magnetic material, between the transmission coil and the reception coil.

FIG. 7A illustrates the relation between the input voltage V_(in) of the IPT converter and the resonant current I_(p) of the primary coil of the resonant tank when the foreign object is intervened. A pulse-shaped signal represents the input voltage V_(in), and a sinusoidal wave signal represents the resonant current I_(p). Also, FIG. 7B shows the relation between the output voltage v_(o,ac) and the current I_(s) induced at in the secondary coil when the foreign object intervenes.

It may be seen that a phase difference Δφ₂ between the input voltage V_(in) and the resonant current I_(p) is different from the phase difference Δφ₁ between the input voltage and the resonant current in the normal case. That is, when the graph of FIG. 6A and the graph of FIG. 7A are compared, it may be seen that the phase difference between the input voltage and the resonant current has changed due to the foreign object interposed between the transmission coil and the reception coil.

That is, when a foreign object exists between the transmission and reception coils, magnetic fluxes may change and parameter values of the resonant network between the coils may also change. The degree of change may be different depending on various variables such as physical properties and size of the foreign object. Accordingly, when the foreign object enters between the transmission and reception coils during the normal operation of the IPT converter 400, a phase difference between the input voltage and the input current of the transmission-side resonant network may be changed.

In this case, the phases of the input voltage and the input current may be matched by controlling the switching frequency according to the changed current phase through the ZPA control.

FIG. 8 is a flowchart for explaining a method of detecting a foreign object between a transmission coil and a reception coil according to an embodiment of the present disclosure.

The foreign object detecting method explained with reference to FIG. 8 may be performed by a WPT control apparatus according to the present disclosure, but an entity performing the method is not limited to the WPT control apparatus.

Referring to FIG. 8, when a ZPA control is started (S801), the input current T_(in) of the IPT converter 400 may be sensed (S802). The sensing of the input current may be continuously performed over time, thereby deriving a moving average I_(in_avg) of the input current (S803).

When the moving average of the input current is derived, a switching frequency may be adjusted by comparing the moving average with an upper limit value and a lower limit value of the input current. More specifically, when the moving average I_(in_avg) of the input current exceeds the upper limit value T_(H_bound) (S811), the switching frequency f may be decreased (S821). On the other hand, when the moving average I_(in_avg) of the input current is less than the lower limit value I_(L_bound), the switching frequency may be increased (S822). If the moving average I_(in_avg) of the input current does not exceed the upper limit value I_(H_bound) and is not less than the lower limit value I_(L_bound), that is, if the moving average I_(in_avg) of the input current is between the upper limit value and the lower limit value, the switching frequency f may be maintained (S823).

When the above-described frequency adjustment procedure is completed, it may be determined whether the current moving average I_(in_avg) is less than the I_(ZCS) (S830). When the current moving average I_(in_avg) is less than the I_(ZCS), it may mean that the IPT converter 400 is operating normally. Then, it may be determined whether the change in the switching frequency is equal to or greater than a predetermined threshold value.

In other words, it may be determined whether a change rate |M_(n)−M_(n-β)| of a moving average of the switching frequency, which is represented by a difference between a moving average M_(n) of the switching frequency in a current period (i.e., n-th period) and a moving average M_(n-β) of the switching frequency in the previous period (i.e., (n−β)-th period), exceeds the predetermined threshold value (S840). As a result of the determination, if the change rate of the switching frequency exceeds the predetermined threshold value, it may be determined that a foreign object is interposed between the transmission coil and the reception coil for WPT and the WPT may be stopped (S851).

More specifically, the foreign object detecting method according to an embodiment of the present disclosure is a method using the change rate of switching frequency in the IPT converter 400. That is, a switching frequency value f_(ri) may be stored every cycle, and a moving average value M_(n) for a plurality of frequency values for a predetermined time may be calculated. If a foreign object intervenes between the transmission pad reception coils, the change rate of the switching frequency may deviate from a certain range (e.g., ‘|M_(n)−M_(n-β)|>α’ in the embodiment of FIG. 8), and it may be determined that foreign object intervenes.

Here, the moving average of the switching frequency according to an embodiment of the present disclosure may be derived through various moving average calculation methods such as a simple moving average, a weighted moving average, and an exponential moving average.

The simple moving average of the switching frequency according to an embodiment of the present disclosure may be calculated according to Equation 1 below. M _(n)=(f ₁ + . . . +f _(n))/n  [Equation 1]

In Equation 1, n represents a corresponding time period. Thus, f₁ represents the switching frequency in a time period 1, and f_(n) represents the switching frequency in a time period n. M_(n) means a moving average value from the time period 1 to the time period n.

Also, the weighted moving average of the switching frequency according to an embodiment of the present disclosure may be calculated according to Equation 2 below. M _(n)=(P ₁ ×f ₁ +P ₂ ×f ₂ + . . . +P _(n) ×f _(n))/(P ₁ +P ₂ + . . . +P _(n))  [Equation 2]

In Equation 2, n represents the corresponding time period. Thus, f₁ represents the switching frequency in the time period 1, and f_(n) represents the switching frequency in the time period n. P₁ is a weight for the time period 1, P₂ is a weight for a time period 2, and P_(n) is a weight for the time period n. M_(n) means a weighted moving average value from the time period 1 to the time period n.

Also, the exponential moving average of the switching frequency according to an embodiment of the present disclosure may be calculated according to Equation 3 below. M _(n) =ν×f _(n)+(1−ν)×M _(n-1),ν=2/(n+1)  [Equation 3]

In Equation 2, n represents the corresponding time period. Thus, f₁ represents the switching frequency in the time period 1, and f_(n) represents the switching frequency in the time period n. The coefficient ν means a degree of weight reduction and represents a constant smoothing factor between 0 and 1.

Here, M_(n) means a moving average from time the time period 1 to the time period n, and M_(n-1) means a weighted moving average from a time period 0 to the time period (n−1).

When the change rate of the switching frequency is less than the predetermined threshold value, it may mean that the IPT converter 400 is operating normally. Therefore, it may be checked whether the battery charging is completed (S850), and the above steps S802 to S840 may be repeatedly performed until the battery charging is completed. When the battery charging is completed, the WPT may be stopped (S851).

On the other hand, if the current moving average I_(in_avg) is equal to or higher than the I_(ZCS) after the frequency adjustment procedure is completed, the WPT may also be stopped (S851). Here, I_(ZCS) may mean a current value of a current sensing point at which a ZCS operation is performed. The ZVS and ZCS may occur according to a phase difference between the input voltage V_(in) and the input current I_(in) which is a resonant current. In order for the WPT system to operate normally, the ZVS operation should be performed. When the magnitude of I_(in) at the current sensing point is larger than 0, it may mean that the ZCS operation is being performed, and thus the WPT may be stopped. The I_(ZCS) may have a preset threshold, for example, a value of 1 A (e.g., a value close to 0 and having a certain margin in actual implementation).

FIG. 9 is a conceptual diagram for explaining a method of detecting a foreign object between a transmission coil and a reception coil according to another embodiment of the present disclosure.

In the foreign object detecting method according to another embodiment of the present disclosure, a foreign object between the transmission and reception coils may be detected using a current value sensed at a current sensing point every period during the ZPA control.

More specifically, referring to FIG. 9, the input current I_(in) of the resonant network illustrated in FIG. 4 may be sensed for predetermined time periods to calculate and store the moving average value I_(in_avg). When the magnitude of I_(in_avg) exceeds a constant current range (i.e., allowable current range) as illustrated in FIG. 9, it may be determined that a foreign object exists, and the WPT may be stopped.

Referring to FIG. 9 illustrating that the input voltage V_(in) and the input current I_(in) for a period during which the switching control is performed, shapes of input currents I_(in-FOD) sensed in the presence of the foreign object are compared with a shape of the input current I_(in) in the normal case, and it may be seen that the input current I_(in-FOD) is higher or lower than the input current I_(in) in the normal case, and exceeds the allowable current range from the input current I_(in) in the normal case.

Here, the sensing of the input current I_(n) may be performed in synchronization with a turn-on time of the switching. However, since a distortion may occur during the current sensing due to a noise generated during the switching, a time point t₁ at which the input current is sensed may be, for example, 100 to 300 nanoseconds (ns) later than a time point to which is the turn-on time of the switching.

FIG. 10 is a flowchart for explaining a method of detecting a foreign object between a transmission coil and a reception coil according to another embodiment of the present disclosure.

The foreign object detecting method explained with reference to FIG. 10 may be performed by a WPT control apparatus according to the present disclosure, but an entity performing the method is not limited to the WPT control apparatus.

Referring to FIG. 10, when a ZPA control is started (S1001), the input current I_(in) of the IPT converter 400 may be sensed (S1002). The sensing of the input current may be continuously performed over time, thereby deriving a moving average I_(in_avg) of the input current (S1003). Here, the moving average of the input current according to an embodiment of the present disclosure may be derived through various moving average calculation methods such as a simple moving average, a weighted moving average, and an exponential moving average.

When the moving average of the input current is derived, a switching frequency may be adjusted by comparing the moving average with an upper limit value and a lower limit value of the input current. More specifically, when the moving average I_(in_avg) of the input current exceeds the upper limit value I_(H_bound) (S1011), the switching frequency f may be decreased (S1021). On the other hand, when the moving average I_(in_avg) of the input current is less than the lower limit value I_(L_bound), the switching frequency may be increased (S1022). If the moving average I_(in_avg) of the input current does not exceed the upper limit value I_(H_bound) and is not less than the lower limit value I_(L_bound), that is, if the moving average I_(in_avg) of the input current is between the upper limit value and the lower limit value, the switching frequency f may be maintained (S1023).

When the above-described frequency adjustment procedure is completed, it may be determined whether the current moving average I_(in_avg) is less than the I_(ZCS) (S1030). When the current moving average I_(in_avg) is less than the I_(ZCS), it may mean that the IPT converter 400 is operating normally, and it may be determined whether a change of the moving average I_(in_avg) falls with a predetermined range (S1040).

In other words, it may be determined whether the moving average I_(in_avg) of the input current is within the predetermined range (i.e., I_(min)≤I_(in_avg)≤I_(max)) (S1040). Here, I_(min) and I_(max) may be reference values configured for the foreign object detection, and define upper and lower limits of a typical input current in the case where a foreign object is not involved.

As a result of the determination, if the moving average value of the input current is out of the predetermined range, it may be determined that a foreign object is interposed between the transmission and reception coil for WPT, and the WPT may be stopped (S1051).

When the moving average I_(in_avg) of the input current falls within the predetermined range, it may mean that the IPT converter 400 is operating normally. Therefore, it may be checked whether the battery charging is completed (S1050), and the above steps S1002 to S1040 may be repeatedly performed until the battery charging is completed. When the battery charging is completed, the WPT may be stopped (S1051).

On the other hand, if the current moving average I_(in_avg) is equal to or higher than the I_(ZCS) after the frequency adjustment procedure is completed, the WPT may also be stopped (S1051).

FIG. 11 is a block diagram illustrating a WPT apparatus according to an embodiment of the present disclosure.

Referring to FIG. 11, a WPT apparatus according to an embodiment of the present disclosure may comprise an IPT converter 400 and a WPT control apparatus 200.

The WPT control apparatus 200 may comprise at least one processor 210 and a non-transitory memory 220 for storing at least one instruction executed by the at least one processor 210.

Also, execution of the at least one instruction stored in the non-transitory memory 220 in accordance with one embodiment of the present disclosure may cause the at least one processor 210 to perform a ZPA control for the IPT converter 400, detect a characteristic parameter with respect to at least one of the input current and the switching frequency of the IPT converter 400, and determine whether or not a foreign object exists between transmission and reception coils according to whether the characteristic parameter deviates from an allowable range.

The methods according to embodiments of the present disclosure may be implemented as program instructions executable by a variety of computers and recorded on a computer readable medium. The computer readable medium may include a program instruction, a data file, a data structure, or a combination thereof. The program instructions recorded on the computer readable medium may be designed and configured specifically for an exemplary embodiment of the present disclosure or can be publicly known and available to those who are skilled in the field of computer software.

Examples of the computer readable medium may include a hardware device including ROM, RAM, and flash memory, which are configured to store and execute the program instructions. Examples of the program instructions include machine codes made by, for example, a compiler, as well as high-level language codes executable by a computer, using an interpreter. The above exemplary hardware device can be configured to operate as at least one software module to perform the operation of the present disclosure, and vice versa.

While some aspects of the present disclosure have been described in the context of an apparatus, it may also represent a description according to a corresponding method, wherein the block or apparatus corresponds to a method step or a feature of the method step. Similarly, aspects described in the context of a method may also be represented by features of the corresponding block or item or corresponding device. Some or all of the method steps may be performed by (or using) a hardware device such as, for example, a microprocessor, a programmable computer, or an electronic circuit. In various exemplary embodiments, one or more of the most important method steps may be performed by such an apparatus.

In embodiments, a programmable logic device (e.g., a field programmable gate array (FPGA)) may be used to perform some or all of the functions of the methods described herein. In embodiments, the FPGA may operate in conjunction with a microprocessor to perform one of the methods described herein. Generally, the methods are preferably performed by some hardware device.

For convenience in explanation and accurate definition in the appended claims, the terms “upper”, “lower”, “internal”, “outer”, “up”, “down”, “upper”, “lower”, “upwards”, “downwards”, “front”, “rear”, “back”, “inside”, “outside”, “inwardly”, “outwardly”, “internal”, “external”, “internal”, “outer”, “forwards”, and “backwards” are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures.

The foregoing descriptions of specific \embodiments of the present disclosure have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the disclosure to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The embodiments were chosen and described to explain certain principles of the disclosure and their practical application, to enable others skilled in the art to make and utilize various embodiments of the present disclosure, as well as various alternatives and modifications thereof. It is intended that the scope of the disclosure be defined by the Claims appended hereto and their equivalents. 

What is claimed is:
 1. A method for detecting a foreign object between a transmission pad and a reception pad for wireless power transfer (WPT), the method comprising steps of: performing a zero phase angle (ZPA) control for a power transfer converter included in or connected to the transmission pad and; detecting a characteristic parameter with respect to at least one of an input current and a switching frequency of the power transfer converter; and determining whether or not a foreign object exists between the transmission pad and the reception pad according to whether the characteristic parameter with respect to at least one of the input current and the switching frequency deviates from an allowable range, wherein the characteristic parameter is a change rate of a moving average of the switching frequency of the power transfer converter.
 2. The method according to claim 1, wherein, in the step of determining, the foreign object is determined to exist between the transmission pad and the reception pad when the change rate of the moving average of the switching frequency is equal to or greater than a first threshold.
 3. The method according to claim 1, wherein the moving average is a simple moving average, a weighted moving average, or an exponential moving average.
 4. The method according to claim 1, wherein the characteristic parameter additionally includes a moving average of the input current which is sensed at a current sensing.
 5. The method according to claim 4, wherein, in the step of determining, the foreign object is determined to exist between the transmission pad and the reception pad when the moving average of the input current deviates from the allowable range.
 6. The method according to claim 1, further comprising a step of, in response to determining that the foreign object exists between the transmission pad and the reception pad, controlling the power transfer converter to make the WPT be stopped.
 7. The method according to claim 1, wherein the ZPA control is a control that causes an input voltage and the input current of the power transfer converter to be in-phase.
 8. A wireless power transfer (WPT) control apparatus comprising at least one processor and a non-transitory memory storing at least one instruction executed by the at least one processor, wherein execution of the at least one instruction in the non-transitory memory causes the at least one processor to: perform a zero phase angle (ZPA) control for a power transfer converter included in or connected to a transmission pad; detect a characteristic parameter with respect to at least one of an input current and a switching frequency of the power transfer converter; and determine whether or not a foreign object exists between the transmission pad and a reception pad according to whether the characteristic parameter with respect to at least one of the input current and the switching frequency deviates from an allowable range, wherein the characteristic parameter is a change rate of a moving average of the switching frequency of the power transfer converter.
 9. The WPT control apparatus according to claim 8, wherein the foreign object is determined to exist between the transmission pad and the reception pad when the change rate of the moving average of the switching frequency is equal to or greater than a first threshold.
 10. The WPT control apparatus according to claim 8, wherein the moving average is a simple moving average, a weighted moving average, or an exponential moving average.
 11. The method according to claim 8, wherein the characteristic parameter additionally includes a moving average of the input current.
 12. The WPT control apparatus according to claim 11, wherein the foreign object is determined to exist between the transmission pad and the reception pad when the moving average of the input current deviates from the allowable range.
 13. The WPT control apparatus according to claim 8, wherein the at least one processor is further configured to control the power transfer converter to make the WPT be stopped in response to determining that the foreign object exists between the transmission pad and the reception pad.
 14. A wireless power transfer (WPT) apparatus comprising: a power transfer converter included in or connected to a transmission pad for WPT, the power transfer converter converting an input voltage to an output voltage in an induction power transfer (IPT) manner, and outputting the output voltage; at least one processor; and a non-transitory memory storing at least one instruction executed by the at least one processor, wherein execution of the at least one instruction in the non-transitory memory causes the at least one processor to: perform a zero phase angle (ZPA) control for the power transfer converter; detect a characteristic parameter with respect to at least one of an input current and a switching frequency of the power transfer converter; and determine whether or not a foreign object exists between the transmission pad and a reception pad according to whether the characteristic parameter with respect to at least one of the input current and the switching frequency deviates an allowable range, wherein the characteristic parameter is a change rate of a moving average of the switching frequency of the power transfer converter.
 15. The WPT apparatus according to claim 14, wherein the foreign object is determined to exist between the transmission pad and the reception pad when the change rate of the moving average of the switching frequency is equal to or greater than a first threshold.
 16. The WPT apparatus according to claim 14, wherein the moving average is a simple moving average, a weighted moving average, or an exponential moving average.
 17. The WPT apparatus according to claim 14, wherein the at least one processor is further configured to control the power transfer converter to make the WPT to be stopped in response to determining that the foreign object exists between the transmission pad and the reception pad. 